Re-filter launcher when a window changes identity

Summary:
LibreOffice reuses the same main window for both its start center and
sub-apps like Writer launched from it, changing the window metadata on
the fly. This ensures we hide e.g. a launcher for Writer when picking
it in the LO start center.

Reviewers: broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D15458